1. Field of the Invention
This invention relates generally to a fuel cell that reduces voltage losses as a result of voltage cycling and, more particularly, to a fuel cell system that includes a bipolar plate that separates the plate flow field in the active area of the fuel cell into a primary domain and a secondary domain so as to reduce the voltage cycling of the fuel cell.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of flow field plates or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of each MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of each MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells from one cell to the next cell and out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It has been discovered that a typical fuel cell stack will have a voltage loss or degradation over the lifetime of the stack. It is well known that the fuel cell stack degradation is, among other things, a result of voltage cycling of the stack. The voltage cycling occurs when the platinum catalyst particles used to enhance the electrochemical reaction transition between an oxidized state and a non-oxidized state, which causes dissolution of the particles. If the voltage of the fuel cell stack is less than about 0.8 volts, the platinum particles are not oxidized and remain a metal. When the voltage of the fuel cell stack goes above about 0.8 volts, the platinum particles begin to oxidize. A low load on the stack may cause the voltage output of the fuel cell stack to go above 0.8 volts. The 0.8 volts corresponds to a current density of 0.2 A/cm2, depending on the power density of the MEA, where a current density above this value does not change the platinum oxidation state. The oxidation voltage threshold may be different for different stacks and different catalysts.
When the platinum particles transition from a metal state to an oxidized state, the oxidized ions in the platinum are able to move from the surface of the MEA towards and probably into the membrane. When the particles convert back to the metal state, they are not in a position to assist in the electrochemical reaction, thus reducing the active catalyst surface and resulting in the voltage degradation of the stack.
The known flow field design of the bipolar plates in the stack causes the reactant gases to be uniformly distributed over the entire active area of the fuel cell so as to homogeneously utilize the catalyst for a maximum power output. In applications where the power demand of the stack is dynamic, a dynamic change of cell voltage occurs. This dynamic change of cell voltage has a significant impact on the aging of the cells because of the voltage degradation.
FIG. 1 is a graph with number of voltage cycles on the horizontal axis and normalized platinum surface area on the vertical axis showing that as the number of voltage cycles between the oxidation state and the metal state increases, the platinum surface area decreases causing the voltage degradation of the stack. The degradation will be different for different types of catalysts, including catalysts of different particle sizes, concentrations and compositions.